The quadrupole mass filter of W. Paul et al. described in U.S. Pat. No. 2,939,952, issued June 7, 1960, consists of four substantially parallel hyperbolic sheet electrodes (or cylindrical rods), symmetrically disposed about an axis. Opposite rods are electrically connected. On one pair of electrically connected oppositely disposed electrodes a dc voltage, U, and an ac voltage of amplitude, V, are placed. On the other pair of electrically connected oppositely disposed electrodes identical voltages, except having an electrical polarity opposite to the first pair, are placed. With proper settings of the dc voltage and the amplitude of the ac voltages, ions of a given charge-to-mass ratio have stable trajectories and oscillate about the axis whereby they do not collide with the electrodes; ions of other than the give charge-to-mass ratio are on unstable trajectories whereby they strike the electrodes. If ions are injected along the axis of the electrode structure, those with the given charge-to-mass ratio do not strike the electrodes and emerge from the opposite end of the electrode structure; however, ions with other than the given charge-to-mass ratio are accelerated in the transverse directions so that they collide with the electrodes and therefore do not emerge from the opposite end of the electrode structure. In this manner, the electrode structure functions as an ion "mass filter."
As noted in U.S. Pat. No. 3,129,327 to W. M. Brubaker of Apr. 14, 1964, an ion entering the electrode structure must pass through fringe fields near and beyond the end of the electrode structure. The ions must also pass through a similar fringe field in emerging from the opposite end of the electrode structure. As pointed out in the aforesaid patent of W. M. Brubaker, the ratio of the dc field strengths to ac field strength in the fringe fields are the same as in the electrode structure itself. Also, as disclosed in the aforesaid patent, an ion of the given charge-to-mass ratio, which is stable within the electrode structure proper, is on an unstable trajectory when it is in the fringe fields. Thus, although an ion would be stable within the electrode structure proper, it may not be received in the electrode structure proper due to its unstable trajectory in the fringe fields. This greatly reduces the transmission of ions of a given charge-to-mass ratio due to their rejection while within the fringe fields.
U.S. Pat. No. 3,129,327 further teaches that the ion trajectories can be stabilized on passage through the fringe fields provided that the ratio of the dc voltage (U) to the ac voltage amplitude (V) is reduced to a lower value than appropriate for use within the electrode structure proper. The aforesaid patent indicates several ways in which this can be accomplished in the case of quadrupole mass filters which have conventional metal electrodes. But the patent does not address itself to the broader problem of spatial separation of ac and dc fields emanating from the same metallic electrodes.